1. Field of the Invention
The present invention relates to novel transferred electron device microwave burst and pulse generator devices which are triggered by optical pulses having durations on the order of a picosecond.
2. Description of the Prior Art
When certain direct gap semiconductor materials such as Gallium Arsenide (GaAs) and Indium Phosphide (InP) are subjected to sufficiently high electric fields, the velocity of the charge carriers decreases with a further increase in field strength (negative differential conductivity). The critical value for the electric field strength E.sub.T required to produce this phenomenon is a characterisistic of the particular semiconductor material. For example, the critical value for Gallium Arsenide is approximately 3.5 kV/cm.
In suitably configured two terminal devices, this negative differential conductivity under critical field strength conditions leads to the formation of a localized space-charge domain at the negatively-biased end of the device which propagates toward the positively-biased end of the device at a characteristic velocity V.sub.d which is approximately 10.sup.7 cm/sec for most materials. The presence of a domain is signaled by a drop in the current flowing through the device. When the domain reaches the positive end of the device, the current rises until a new domain forms which subsequently propagates through the device resulting in another drop in current. Thus, an oscillator device can be formed which oscillates at a frequency f.sub.o equal to the characteristic velocity V.sub.d divided by the propagation length of the device. Such devices are known as transferred electron devices (TED) or Gunn effect devices.
Typical propagation lengths for microwave integrated circuit TED's range from five to several hundred microns, and oscillation frequencies therefore extend from several hundred megahertz to over twenty gigahertz. Specially constructed devices can oscillate at frequencies beyond 100 GHz.
A TED to which control electrodes, in addition to the anode and cathode electrodes, are attached is known as a transferred-electron logic device (TELD). FIG. 1(a) illustrates a TELD 10 which includes a semiconductor substrate 12, an anode region 14, a cathode region 16, and a Schottky-barrier gate electrode 18. A depletion layer 22, under the gate electrode 18 is swept free of carriers by a built-in field and therefore constricts the volume of semiconductor material available for current flow. Thus, the electric field located under the gate electrode 18 is higher than elsewhere within the TELD 10 as shown in FIGS. 1(b) through 1(d). The maximum value of the electric field E.sub.g is located under the edge of the gate electrode 18 which is closest to the anode region 14 as shown in these Figures. The position of the maximum electric field E.sub.g is the domain nucleation site: i.e., the site at which domains form. The thickness of the depletion layer 22, and thus the magnitude of the electric field, can be changed by applying and/or changing a control voltage V.sub.g applied to the gate 18. The device can therefore be biased below the critical electric field E.sub.T with a fixed bias voltage V.sub.a as shown in FIGS. 1(a ) and 1(b) and can be triggered into operation with a negative voltage pulse V.sub.g (t) applied to the gate as shown in FIGS. 1(a), 1(c), and 1(d).
In most devices of this type, only one domain, such as domain 20 in FIG. 1(a), is present at a time and a new domain is formed after an existing domain is annihilated at the anode. If the gate is held at a sufficiently negative potential, the device will oscillate at a fixed frequency f.sub.o =V.sub.d /L.sub.ga (as detected across the load resister 54) where V.sub.d is the characteristic velocity and L.sub.ga is the gate to anode distance. If a sufficiently short voltage pulse is applied to the gate in place of a constant voltage, a single domain of duration T.sub.o =l/f.sub.o will result.
It is known in the art that a single domain or a pair of domains may be formed in a two-terminal TED, biased slightly below the critical field level E.sub.T which is subjected to illumination between the cathode and anode electrodes. For example, Adams and Schulte (Applied Physics Letters, Vol. 15, No. 8, Oct. 15, 1969) determined that the application of a 1 nanosecond optical pulse to such a device will trigger the formation of a single domain having a transit time proportional to the distance between the cathode and the point of illumination. Other experimenters have observed a pair of domains under similar conditions.
In each of the prior art TED and TELD devices described above thermal carrier density fluctuations produce a randomly fluctuating electric field noise component E.sub.n which limits the speed of response of these devices. Also, in voltage triggered devices, the gate voltage pulses have a risetime limited to the order of 100 picoseconds or more by the bandwidth of the associated circuitry (the pulse source plus transmission lines). The resulting finite rise time of the gate electric field dE.sub.g /dt and the field noise E.sub.n together result in an uncertainty in the time .DELTA.t.sub.n at which E.sub.g rises above E.sub.T, given by: ##EQU1## This uncertainty in the triggering time is known as "fm noise". For typical microwave integrated circuit TELD's .DELTA.t.sub.n lies in the range of 40 to 100 picoseconds.
The present invention relates to novel transferred-electron device oscillators which are triggered by optical pulses having durations on the order of a picosecond. Optical pulses of picosecond duration can trigger such devices much more quickly than voltage pulses, with a drastic reduction in fm noise as a result. Bursts of oscillations of independently controllable duration and frequency may be generated without complicated electronic circuitry and single current pulses, useful in logic applications, may also be produced with picosecond time synchronization.
Picosecond optical pulses may be obtained from a variety of sources. For example, synchronously mode-locked linear or ring dye lasers generate extremely short pulses; however. these devices are physically large. Solid state lasers with external cavity mirrors are a more compact source. Some solid-state lasers without external resonators exhibit "self-pulsing" and are another potential source. Light-emitting diodes and solid-state lasers may also be electronically driven to produce picosecond pulses.